Security is an important concern in the realm of digital production and/or reproduction of documents. Known digital image printing/copying systems produce documents of such high quality that a need has been identified to prevent effective printing/copying of certain documents such as high-value printed items including tickets, financial instruments, security ID badges, and the like. Known techniques include printing the original document in a manner such that it includes a digital “watermark” using only conventional paper and toner or ink. A digital watermark in a broad sense can be defined as information, for example one or more letters, words, symbols or patterns, that is at least partially (preferably fully or at least substantially) hidden in a printed image under normal viewing conditions but that is more clearly discernible under certain specialized viewing conditions. Unauthorized reproduction of documents including such digital watermarks typically degrades or obscures the digital watermark, which can aid in detection of counterfeit documents.
Methods and systems are known for including infrared (IR) marks in printed documents using conventional papers and ordinary inks/toners (e.g., CMYK ink/toner) by using metameric colorant mixtures. Under visible lighting conditions, the different metameric colorant mixtures that are printed on respective adjacent portions of the paper together define an overall printed document region that appears substantially uniform in color. Under infrared (IR) lighting (e.g., electromagnetic radiation wavelengths longer than 700 nm but shorter than microwaves), these different colorant mixtures exhibit different IR absorption/transmission and, thus, different suppression of IR reflectance from the paper or other substrate on which the colorants are printed. The region printed with the colorant mixture that absorbs less (transmits more) IR appears as a lighter/brighter region while the adjacent area printed with the colorant mixture that absorbs more (transmits less) IR appears as a darker region. These contrast variations under IR lighting are used to create watermark-like patterns, e.g., numbers, letters, symbols, shapes, that can be observed using an IR camera or other IR sensor/imaging device.
An example of the above IR mark method is shown in FIG. 1, wherein a colorant mixture “B” is selected and applied to patch area BP which, in this example, is shaped as the alphanumeric symbol “0”. Further, a colorant mixture “A” is selected and applied to patch area AP arranged here in substantially close spatial proximity to patch area BP, and thereby providing a background around patch area BP. The patch areas AP and BP together define a security mark region SMR. Both colorant mixture A and colorant mixture B are comprised of one or more suitably selected colorants, but colorant mixtures A and B are different mixtures. In the illustrated example, colorant mixture A is selected to provide greater IR absorption (less IR transmission) and greater substrate IR reflectance suppression as compared to colorant mixture B. The colorant mixtures A and B will also be selected to match each other closely in their average color and luminance when viewed under visible light conditions. As shown at IR in FIG. 1, under IR lighting conditions, patch BP will appear brighter as compared to patch AP to define a security mark SM, due to the relatively high transmission of IR in the region BP leading to an increased IR reflectance from the paper substrate as compared to the region AP that has a relatively high absorption of IR and corresponding decrease in IR reflectance from the paper substrate. In contrast, under visible light conditions as shown at VIS, patches AP, BP are at least substantially indistinguishable. This property of matching color under a first lighting condition (e.g., visible light) but unmatched color under a second lighting condition (e.g., IR) is referred to as metamerism and the colorant mixtures A and B can be said to define a metameric or approximately metameric pair.
Additional details relating to infrared marks are disclosed in U.S. Patent Application Publication No. 2008/0305444 (application Ser. No. 11/758,359 filed Jun. 5, 2007) in the name of Reiner Eschbach, Raja Bala, and Martin Maltz and entitled “Infrared Encoding of Security Elements Using Standard Xerographic Materials with Distraction Patterns” and in U.S. Patent Application Publication No. 2008/0302263 (Ser. No. 11/758,344 filed Jun. 5, 2007) in the name of Reiner Eschbach, Raja Bala, and Martin Maltz and entitled “Infrared Encoding of Security Elements Using Standard Xerographic Materials” and in U.S. Patent Application Publication No. 2009/0262400 (application Ser. No. 12/106,709 filed Apr. 21, 2008) in the name of Reiner Eschbach and Martin Maltz and entitled “Infrared Watermarking of Photographic Images by Matched Differential Black Strategies,” and the entire disclosure of each of these prior applications is hereby expressly incorporated by reference into the present specification.
It has been deemed highly desirable to include variable IR mark data in a printed document in real time before the document is printed such that the IR mark encodes information that is unique to each particular document occurrence or a series of document occurrences. As an example, it would be desirable to embed a particular encoded number sequence in a printed document that matches a visible number sequence printed on the document when the visible number sequence changes for each occurrence of the printed document. As such, the document could be verified by ensuring that the IR mark matches the visible number sequence. While known methods have allowed for variable IR marks to be included in solid color regions, a need has been identified for a new and improved method for including variable IR marks efficiently and effectively on a real-time basis into photographic and other variable image data in which the image data does not present a region of uniform color that can be used to include an IR mark using metameric color pairs as noted above.